Photosynthesis: The Breath of Life

First Law of Thermodynamics: Energy cannot be created or
destroyed, but it can be changed in form.

Second Law of Thermodynamics: All systems tend to go from a state
of greater organization to a state of lesser organization with a concommitant loss of usable energy.

Photosynthesis, the conversion of inorganic water (H2O) and carbon dioxide (CO2) into organic sugar is the plant personification of these two Laws. During the process of photosynthesis...

randomizing solar energy intercepted by plants, and
instantaneously changed (transduced) into electrical energy (the kinetic energy of electron flow). It is then "packaged" as chemical energy (the potential energy stored in the covalent bonds of sugar molecules). (First Law)

No energy transformation is 100% efficient. Not all solar
energy captured by a plant cell is converted to electrical and then chemical energy. Some
of it is lost as heat or other randomizing energy that cannot be used to do work (entropy). (Second Law)

Like cellular respiration, photosynthesis proceeds via a series of orderly enzymatic reactions.

Recall Baby Chemistry:

An enzyme is a proteinaceous biological catalyst.

A catalyst is a molecule that increases the rate of a reaction without being consumed or permanently changed by that reaction.

A substrate is a molecule upon which the catalyst acts.

The active site of an enzyme is a (usually) three-dimensional "pocket"
into which a substrate is chemically bonded for catalysis of its particular reaction.

An endergonic reaction absorbs energy as it proceeds.

An exergonic reaction releases energy as it proceeds.

In a catabolic reaction, a substrate is broken down.

In an anabolic reaction, a substance is constructed from one or more substrates.

In an oxidation reaction, a molecule loses one or more electrons, and becomes more positively charged. (A positively charged ion is a cation.)

In a reduction reaction, a molecule gains one or more electrons, and becomes more negatively charged. (A negatively charged ion is an anion.)

In biological systems, oxidation and reduction reactions are most often paired, resulting in a redox reaction: as one molecule is oxidized, the molecule that accepts its electrons is reduced.

Keep these familiar terms in mind as we follow the course of photosynthesis in overview.

The Nature of Light Energy: Physics 101

By now we should all know that
the sun provides most of the earth's energy in the form of electromagnetic radiation.

(Can you think of any energy on earth that's not solar in origin?)

The smallest unit of light energy is known as a quantum, which has properties of both a particle (it can be deflected by solid matter) and a wave (it travels through space in an up and down pattern at a specific wavelength).

Not all quanta are the same. Although they all travel through space at the speed of light (299,792,458 m/sec), they may do so at different wavelengths...

...and different frequencies.

Different wavelengths of electromagnetic energy correspond to different physical entitles which react with matter in different ways.

Visible Light: Quanta that Stimulate your Central Nervous System

The quanta that we can perceive as light are called photons, and photons of different wavelengths comprise the visible spectrum.

Humans can see photons ranging in wavelength from about 380 nm (violet) to about 700 nm (red), and a photon will be perceived by your brain as a certain color depending on its wavelength when it hits the color-sensing photoreceptors of your retina.(Note: A nanometer = 10-9 meters, or 0.000000001 meters.)

The shorter the wavelength, the higher the frequency, and the higher the energy.

The highest energy photons are in the violet region; the lowest energy ones are in
the red region

The Wonder of Photosynthesis

Recall that an autotroph (auto = "self" and troph = "feeding") is an organism that captures energy and stores it in the chemical bonds of organic molecules that it manufactures from inorganic molecules. They are also known as producers or primary producers. The greatest autotroph biomass on earth is comprised of plants.

(A heterotroph (hetero = "other" and troph = "feeding") is an organism that eats other organisms to obtain energy. They are also known as consumers.)

The most common means by which autotrophs make organic molecules (sugar) is via photosynthesis.

(Autotrophs that capture light energy are called photoautotrophs, though there are other kinds of autotrophs.)

Plants are photoautotrophs that absorb photons only in a specific region of the spectrum.
A pigment is any substance that absorbs light. The main pigments responsible for the initiation of photosynthesis are chlorophylls and carotenoids which absorb light in different regions of the visible spectrum. And these pigments, as you already know are embedded in the thylakoid membranes of the chloroplasts.

Photons interact with matter--including plant pigments--in one of three ways. A photon striking matter (liquid, gas or solid) can be

transmitted (it passes through the matter)

reflected (it bounces off the matter and changes direction)

absorbed (its energy is converted into the energy of the molecule it hits)

Only when absorbed can photons initiate biological activity.

Plant pigments absorb photons in the violet/blue region and in the red region.

Thus, only violet, blue and red light will drive photosynthesis.

All other wavelengths are reflected, which is why plants look green. They are reflecting or transmitting the green light, not using it to make sugar.

You should already know the overall chemical reaction of photosynthesis:

It takes...

six molecules/mole of carbon dioxide (0.037% of the atmosphere) plus

12 molecules/mole of water

in the presence of light and the proper enzymes in the cell, to make

one molecule/mole of glucose

6 molecules/moles of oxygen

6 molecules/moles of water

The sugar (glucose) is the storage form for energy in plants, and it's often converted into long chains for long-term storage as carbohydrate, which forms the body of the plant. The oxygen and water are side products that are not used by the plant in this reaction.

Why Photosynthesis?

What does the plant do with the sugar molecules, once it has them?

body structure (cellulose and other macromolecules)

energy storage (for cellular work)

The latter, of course, is done via cellular respiration, the overall chemical equation for which is exactly the opposite of photosynthesis:

It takes...

one molecule/mole of glucose in the presence of

six molecules/moles of oxygen and

six molecules/moles of water

can be "burned" to release stored energy as well as the "waste" products of

6 molecules/moles of carbon dioxide and

12 molecules/moles of water

Chlorophyll

All photoautotrophs use an isomer of chlorophyll known as chlorophyll a as the primary photosynthetic pigment. This is the only type of chlorophyll that can pass excited electrons to the primary electron acceptor protein in the thylakoid membrane. But there are other isomers of chlorophyll. The one present in all members of the monophyletic taxon known as Viridaeplantae also contain chlorophyll b and carotenoids as accessory pigments.

The accessory pigments cannot pass excited electrons to the PEA protein, but they can pass them to the "team captain," chlorophyll a. What do you suppose might be the advantage of accessory pigments? Consider...

Isolated chlorophyll molecules in solution and exposed to light will absorb light, resulting in an excitation of the chlorophyll's electrons. In a live cell, the excited electron would be sent along a transport chain, and its energy harvested in a stepwise fashion.

The isolated chlorophylls, with no place to send their electrons, exhibit a phenomenon known as fluorescence. (And if you didn't come to class for the explanation, you're just going to have to read and figure this out on your own. Hint: Check out your old BIL 151 lab on Photosynthesis pigments!)

electrons from the water molecules are passed along the electron transport system on the thylakoid membranes

as electrons are passed along the chain of proteins, energy is lost at each transfer, and this is packaged in the high-energy phosphate bonds of ATP.

some of the protons from the split water molecules reacts with another energy courier molecule, NADP (nicotinamide adenine dinucleotide phosphate) to form NADP-H, which stores the energy of the proton.

both ATP and NADP-H shuttle the energy of captured photons, now stored as chemical energy, to the stroma (the thick gel matrix of the chloroplasts) where it will be transduced yet again and stored in the covalent bonds of sugars.

Heart of the Light Reactions: The Photosystems

A photosystem is a light-gathering complex composed of a proteinaceous reaction center complex surrounded by several light-harvesting complexes. These are embedded in the thylakoid membranes.

Each light-harvesting complex consists of various pigment molecules (chlorophyll a, chlorophyll b, carotenoids) bound to proteins in the thylakoid membrane.

The systems are spread out over the surface of the thylakoid, providing a large surface area for light harvesting as well as a variety of pigments with different absorbance spectra.

Antenna pigments, such as carotenoids and chlorophyll b, pass their excited electrons to one another until the e- reaches the reaction center complex.

The reaction center complex contains a pair of chlorophyll a molecules associated with a large protein, the primary electron acceptor.

As the chlorophyll a molecules accept excited electrons from other pigments in the photosystem, they pass them along to the primary electron acceptor (a redox reaction).

The primary electron acceptor passes the excited electron to a small "shuttle" protein that carries it to a cytochrome complex of proteins that form an electron transport chain.

And you know what happens when an excited electron passes along such a chain: at each transfer, energy is lost. But in a live photosystem, it isn't puffed off into space. There are small, energy-courier molecules just waiting to package that energy: ADP and NADP. These are converted to energy-storing ATP and NADP-H, respectively.

The ATP and NADP-H travel to the stroma, where their energy will be packaged in the covalent bonds of sugar.

Photosystems I and II

There are two types of photosystems (named in order of their discovery, not in order of their function) in the thylakoid membranes:

reaction center contains chlorophyll a molecules (P700) that have λmax of 700nm.

The chlorophyll a molecules of the two photosystems are nearly identical, but each is associated with different proteins in their respective reaction centers.

Here's how it works...

A photon is absorbed by a chlorophyll or carotenoid molecule in the thylakoid membrane. As it falls back to its ground state, its energy is transferred to the electron of an adjacent pigment, raising it to an excited state. This continues until the excited electron belongs to the famous P680 chlorophyll of PS II.

The excited P680 electron is transferred to a primary electron acceptor protein. The oxidized P680 is now P680.

Nearby, an enzyme splits water to yield

two electrons - these replace those lost by the two P680 molecules in the reaction center

one oxygen atom - this combines with another oxygen atom from a different split water molecule to form oxygen gas (O2)

two protons (hydrogen ions) - some will combine with NADP (to form NADP-H) to store energy to be shuttled to the stroma for the light-independent reactions.

The excited electrons from PS II travel to PS I via an electron transport chain (similar components as those found in the mitochondrial electron transport chain) consisting of a cytochrome complex known as plastoqinone (Pq) and another protein, plastocyanin (Pc).

As electrons "fall" exergonically from one component of the electron-transport chain to the next, our old pal the Second Law of Thermodynamics rears its head: energy is released at each transfer, but quickly captured and packaged in the high-energy phosphate bonds of ATP. (Electrons passign through teh cyctochrome comlex results in the pumping of protons out of the membrane, and the resulting potential difference is used in chemiosmosis.

Meanwhile, back at PSI, chlorophylls and carotenoids are behaving in a similar manner, doing the wave, and transferring photon energy (not converted to electrical energy--the flow of electrons) to the pair of P700 chlorophylls at the reaction center.

P700 transfers its excited electron to its own primary electron acceptor, and becomes P700+ (redox again!).

The electron reaching the "bottom" of the electron transport chain in PSII is shuttled to PSI, where it replaces the lost electron of P700+, restoring it to its original P700 configuration.

PSI excited electrons travel along a different electron transport chain via the protein ferredoxin ((Fd). There is no proton pump at the PSI electron transport chain, so no ATP is produced there.

A special enzyme, NADP+ reductase, catalyzes the transfer of two electrons from Fd to one NADP+, reducing it to energy-storing NADP-H.

One picture is worth a lot of words.

The flow of electrons from PSII to PSI has been called linear electron flow or non-cyclic electron flow to distinguish it from a less common phenomenon...

Cyclic electron flow

Once in a while, an excited electron from PSI's primary electron acceptor will "short circuit" and pop over to Fd and back into the electron transport chain between PSII and PSI, instead of to NADP+ reductase.

This will produce more ATP (and is a good supplement), but no NADP-H.

It's probably an accident of the proximity of the various molecules, but because it's not deleterious there has been no selection pressure against it. It just happens.

The Calvin Cycle: An Anabolic Cycle

During the light-independent reactions of photosynthesis, a.k.a., The Calvin Cycle, the energy stored in ATP and NADP-H during the light-dependent reactions is briefly released and then repackaged into the covalent bonds of sugar.

The sugar first produced by the Calvin Cycle is not 6-carbon glucose (this is constructed later), but a 3-carbon sugar known as glyceraldehyde-3-phosphate or G3P.

The Calvin Cycle "spins" three times to make this 3-carbon sugar from three molecules of CO2. The conversion of inorganic carbon from CO2 into the carbons of an organic molecule, G3P, is known as carbon fixation.

Remember that there are many Calvin Cycles going on in the stroma at any given time, so many atoms of carbon are constantly being fixed quickly into sugar.

The Calvin Cycle can be reduce to three phases:

I. Carbon Fixation

One CO2 molecule is attached to a 5-carbon sugar named ribulose biphosphate (RuBP) by an enzyme named rubisco. (Rubisco may be the most abundant protein on earth! It is certainly the most abundant protein in the chloroplasts.)

The resulting 6-carbon sugar is extremely unstable, and it immediately splits in half, forming two molecules of 3-carbon 3-phospho glycerate.

II. Reduction

A phosphate group (from ATP) is attached to each 3-phospho glycerate, forming 1,3-phosphoglycerate (guess where the phosphates are attached!)

NADP-H swoops in and reduces 1,3-phosphoglycerate, which loses its phosphate group (in addition to gaining electrons) to become our old pal, G3P.

For every six G3Ps made by the Calvin Cycle, five are recycled back to regenerate new molecules of RuBp. Only ONE leaves the cycle to be packaged for use by the plant.

It takes two molecules of G3P (3 carbons) to make one molecule of glucose (6 carbons).

ADP and NADP+ formed during the Calvin Cycle are shuttled back to the photosystems to be "recharged" with energy, and converted to ATP and NADP-H, respectively.

III. Regeneration of RuBP

Five molecules of G3P from the Calvin Cycle's Reduction phase pass through a complex series of enzymatic reactions to yield three molecules of RuBP. This costs the cell 3 more molecules of ATP, but provides new "machinery" for the Calvin Cycle to continue spinning

What's the cost of making carbohydrate?

To make one molecule (or mole) of G3P, the plant must expend the energy of nine ATP molecules (or moles) (at 7.3kcal/mole) and six molecules of NADP-H. Some of this is packaged in the sugar, of course. But because of our old pal The Second Law of Thermodynamics, some is also lost as entropy.

(A good question might be...How many CO2 molecules, ATP molecules and NADP-H molecules are needed to make one molecule of glucose?)

Plants that produce 3-carbon G3P via the Calvin Cycle are called C3 plants. But there are some plants that have an alternative mechanism for carbon fixation, and this is particularly true in plants that evolved in hot, arid climates.

During the day, CAM plants' stomates are closed. But at night they open, taking up CO2 and incorporating it into a variety of organic acids for storage. These acids are stored in the vacuoles of leaf mesophyll cells until daybreak.

As the light reactions start up in response to light, CO2 is released from the organic acids. The energy from ATP and NADP-H from the light reactions can now be used to fix that carbon, even though the stomates are closed.

C4 and CAM are similar in that they use organic intermediates to store CO2 for later release, concentrating it when the plant isn't open to the atmosphere.

C4 and CAM are different in that C4 plants have two types of cells, and they separate carbon dioxide storage cells (mesophyll) from Calvin Cycle-performing cells (bundle-sheath). CAM plants perform both functions in the same (mesophyll) cells).